Generally, petroleum, coal and/or natural gas are burnt as heat sources for generation of steam and hot water in generating station and factories.
On the other hand, for small scale equipment, an electrical resistance heater may be utilized as a heat source from the point of convenience, although some small scale boilers still utilize petroleum and/or coal as heat sources.
There has been known an another type of electrical heater, that is, a low-frequency electromagnetic induction heater (Japanese unexamined utility model No. 56-86789, and Japanese examined patent No. 58-39525 etc.).
However, there have been serious problems in heating methods which utilize the burning of petroleum, coal and/or natural gas as heat sources. For example, a boiler of the type mentioned above has the problem that scale precipitates tightly to a heating pipe, therefore the thermal conductivity is lowered to cause inefficient heating and finally the heating pipe itself is destroyed due to an undesirably big temperature difference between heater part and the water to be heated. Currently, in order to avoid the occurrence of the above problem, the water to be supplied to a boiler and heated is required to be anti-scale treated before it is supplied by the use of chemicals which have effects in degassing de-oxigenation or maintaining the water in an alkaline range in pH. Further, currently heating system for whole building, which operates by circulating steam generated by burning of petroleum, coal and/or natural gas, are widely used, but they have the problem that the loss of energy is tremendous and the system cannot be regarded as an efficient heating system.
In the case where an electrical resistance heater is inserted and operated in water, water is locally and too strongly heated at high temeperature far from the boiling point of 100.degree. C. Therefore, unless the heater having a sufficient interfacial conduction area is used, various problems unavoidably arise. The problems are summarized below.
(1) Unless the electric power is maintained below 2 watt per 1 cm.sup.2, efficiency of heat conduction from the heater to water is decreased and the heater is destroyed.
(2) Because the required voltage amounts from 200 to 400 V, a very high voltage, sufficient isolation and insulation of heater from water must be provided. Usually an insulating material is a low thermally conductive material, so the heat conduction from heater to water is terribly impeded.
Low thermal conduction from heater to water causes over-heating of the heater, particularly of the heater surface, and when water molecule touch to the heater surface over-heated steam-explosion may occur to cause so called bumping, flashing, and/or forming phenomena. The occurrence of these phenomena can be fatally dangerous and give a fundamental problem that the thermal efficiency is drastically decreased.
A further problem for an electrical resistance heater is that it causes too big a temperature difference between the heating part and water as in the case where the heat source is gas burning. This too big temperature difference induces precipitation and adhesion of inorganic and organic solute components in water to the surface of heater, and because the precipitants behave as heat insulating materials, efficiency of thermal transfer is reduced, and therefore, boiling of water becomes an inefficient process. At the same time, heat release by heater becomes an inefficient process, and it may finally cause a suicidal accident, i.e., breaking of heater wire. In order to avoid this kind of accident, a current heater for water has large surface area, and very long heater is introduced into a water tank. However, still the above type of heater has problems that change of heater for cleaning is very annoying and operation reliability is low.
Further to the above, a fundamental and unimprovable problem for the conventional electrical resistance heaters is that they must have large buffer-water-tank in order to accomplish accurate temperature control of water, and therefore, they can not be miniaturized.
The low-frequency electromagnetic induction heaters disclosed in Japanese unexamined utility model No. 56-86789 or in Japanese examined patent No.58-39525 have problems, so that the design has not yet been optimized, the temperature difference between a heating element and a material to be heated is quite big, and thermal efficiency is not high enough.